american cities, including pittsburgh and new york. man: new york city went to philadelphia and said, "you know, we're thinking of developing a hudson river water supply -- what do you suggest we do?" and they said, "we've had "a lot of problems on the schuylkill. "don't go to the hudson river. go to the upland and work by gravity." and that's what new york city did. they first went to the hudson highlands, but 150 years later, it went to the delaware highlands. and really diverted the water that normally went to philadelphia to new york city. i don't think they anticipated that. narrator: the majority of new york city's drinking water comes from watersheds in upstate new york. a watershed is the area of land where water from rain or snow melt drains downhill into a body of water. mountains act as a funnel to feed rivers and lakes. and in this case, reservoirs. in the new york city system, water is collected and stored in 19 reservoirs, which can hold more than a year's supply -- over 580 billion gallons of water. almost all of the system is fed by gravity, without the use of energy-consum
